Drilling of the seabed is widely conducted for a number of purposes including geotechnical sampling and testing, offshore hydrocarbons exploration, geohazards identification, and specific scientific studies. Such drilling activities can encounter shallow gas deposits in the seabed that can present potentially serious hazards to operations. Seabed gas may originate from decomposition of marine organisms within shallow sedimentary layers or it may seep from deep hydrocarbon sources. Such gas deposits can be toxic and/or explosive and can be confined within the seabed at high pressure.
In certain regimes of high pressure and low temperature, at water depths beyond 300 meters, marine sediments may contain gas hydrates close beneath the sea floor.
Hydrates are quasi-stable solid phase gas-water structures that can significantly influence the strength and stability of the seafloor sediments in which they occur. Gas hydrates are thus an important consideration in offshore geohazards (apart from attracting interest as a potential energy resource), especially in areas where deepwater oil and gas exploration and exploitation activities can alter soil conditions to the extent that rapid destabilization of the seafloor may occur.
In some cases the presence of shallow gas can be recognised by survey prior to commencement of drilling, where pock marks and/or shallow depressions are identified on the seabed. Gas hydrate sediments and underlying free gas may be indicated on seismic records, appearing as a bottom simulating reflector. In other cases, particularly where impervious layers exist in the seabed, the presence of shallow gas deposits may not be immediately evident from seabed features, and thus may be encountered unexpectedly.
Seabed drilling operations may be carried out from a surface platform such as a drillship, jack-up rig or semi-submersible drilling rig, in which case the drillstring extends through a riser in the water column and into the borehole. In a less expensive alternative form of seabed drilling and sampling, operations are carried out via a remotely controlled system, deployed to the seafloor on an umbilical from a surface vessel. In this case the drillstring extends into the borehole only from the seabed rig and the surface vessel need not be stationed directly above the borehole.
Interception of a borehole with a shallow gas deposit may allow release of toxic and/or flammable gas such as hydrogen sulphide and methane which, if vented to the surface near a drilling vessel, can endanger health and safety of personnel and safety of equipment. In the case of drilling equipment supported on the seabed, release of high pressure gas can result in a sudden and uncontrolled loss of seabed bearing strength or possible scouring and undermining of the equipment footings. Such events may destabilise the equipment, with resultant damage and loss of productivity through tilting or toppling. Drilling operations through hydrates can cause pressure and temperature changes which may result in rapid dissociation of the hydrates and consequent blowouts and/or destabilisation of the seafloor.
When samples are taken for geotechnical assessment from seabed sediments in deep water they undergo extreme pressure relief as they are brought to the surface. Gases dissolved in the pore water may come out of solution and cause sample disturbance, which can impact significantly on sample quality and subsequent interpretation of laboratory test results. Knowledge of the strength characteristics of marine sediment soils in which gas hydrate deposits can occur is vital to the economic establishment of seabed infrastructure. It is therefore an important step to know the in situ dissolved gas content and degree of saturation.
Detection, monitoring and measurement of shallow gas occurrence are therefore important aspects of seabed drilling, sampling and geotechnical investigation. In conventional practice this may involve (a) monitoring of drilling returns at the surface and (b) deployment of gas sampling probes in the borehole.
(a) Drilling Returns Monitoring
For drilling operations generally, drilling fluid or mud is pumped to the cutting bit through the drill pipe to cool and lubricate the bit and to remove cuttings from the borehole. The drilling mud returned from the borehole carries with it a continuous sample of material representative of the geological formations being penetrated by the drill bit, including free and dissolved gases released from the soil matrix. The drilling mud ‘returns’ typically flow up the annular passage between the rotating drill pipe and the surrounding casing pipe.
In the form of seabed drilling where operations are carried out from a surface vessel or platform, a mud logging system is typically used. This includes monitoring and analysis of gases liberated from the returned drilling mud before it passes back to the holding tank. Various sensors or high speed gas chromatography instruments measure the presence of hydrogen sulphide and of hydrocarbons, particularly those of low molecular weight such as methane. When operating at great water depth there is however, a significant measurement lag due to the time taken for the drilling mud returns to travel from the borehole to the surface measurement zone. Unexpected interception of a high pressure gas pocket may cause a sudden rise or ‘kick’ in pressure in the drill string and possible gas blow-out in extreme cases, necessitating the use of blow-out prevention equipment.
In the form of seabed drilling where operations are carried out via a remotely operated system, the drilling fluid may be seawater drawn from the immediate surrounds, or seawater mixed to a desired ratio with a synthetic mud concentrate, prior to pumping down the drillstring to the cutting bit. In this case the drilling mud is not recycled, but discharged at the seafloor together with the cuttings from the borehole. Such remotely operated seabed systems are not commonly equipped with means for blow-out prevention and are currently disadvantaged in lacking gas monitoring capability. They are therefore unable to detect whether the borehole may be approaching or intersecting shallow gas deposits, or to forewarn the drilling operator that a potentially unsafe condition is developing.
(b) Gas Sampling Probes
Sampling probes such as the NGI Deepwater Gas Probe are conventionally used to obtain samples of in situ pore water that can be analysed for content of gas. These probes have an internal container that can be opened and closed to seal off a pore water sample, together with temperature and pressure logging instrumentation. There is however no means of communication with the probe during the test, which gives rise to a number of disadvantages in that no data is available in real time; logged measurements must await retrieval of the probe back to the surface; required sampling times and sampling intervals must be pre-programmed prior to launch, based on an assumed knowledge of waiting time and soil conditions. The lack of in situ measurement capability requires on-board laboratory facilities and contributes further delay while results are obtained from separate instrumental analysis of the pore water gas content.
Another form of sampling, with particular application in the case of gas hydrates, involves the use of pressurised coring tools such as the HYACE Rotary Corer and the Fugro Pressure Corer. Gas hydrates are naturally occurring unstable compounds that rapidly dissociate at normal atmospheric pressure. Pressurised tools allow samples to be autoclaved and brought intact to the surface at their natural in situ pressure for various physical measurements and geochemical analysis. While useful for ground truthing and other studies, pressurised corers are currently limited to large diameter tools unsuitable for deployment via remotely operated seabed systems.
As used herein, the phrase ‘remotely operated seabed system’ generally refers to the situation where the drilling tools and/or downhole probes are deployed robotically or otherwise down the borehole from a seabed platform or other type of vehicle rather than manually from a surface platform. Communication from the probe to the seabed platform/system may be by wire(s), cable(s) and/or by wireless means. Communication between the seabed system and the surface vessel (remote operator station) is by wire and/or cable (e.g. electrical or optical fibre telemetry).
It is an object of the present invention to provide methods and/or apparatus which alleviates one or more of the above described disadvantages associated with detection, monitoring and sampling of seabed gas.